In previously known mechanisms of this kind it has been considered necessary to have one actuating mechanism at each end of the pivoted sideboard, and also a separate locking mechanism for each end of the pivoted sideboard. The known mechanisms of this kind do, however, suffer from considerable drawbacks relating to the installation thereof, since the work with installing two separate actuating mechanisms and two separate locking mechanisms, possibly together with the requisite remote-controlling equipment, demands a great effort of highly qualified personnel, which situation is not made more tolerable by the fact that in many cases the work has to be done out-of-doors during inclement weather. As to the functioning, the known mechanisms are relatively vulnerable, especially because one of the actuating mechanisms is placed in a highly exposed position at the rear of the vehicle in question, and both this mechanism, the associated locking mechanism as well as the remote-controlling equipment may easily be damaged under the rough operating conditions often encountered by vehicles of the kind presently referred to, namely during work on building and construction sites.